This invention relates to devices, particularly optical devices, for controlling propagation of energy, particularly optical beams, using electric field control. In particular, the invention relates to devices with poled structures, including periodically poled structures, and electrodes which permit controlled propagation of optical energy in the presence of controlled electric fields applied between electrodes.
More particularly, the invention relates to a new class of switchable energy conversion devices, energy guiding devices, filters, and bulk energy transfer devices based on the use of poled structures in solid state material. In some applications, the poled structures can be switched electrically to control optical or even acoustic energy. A poled switch is especially applicable to the fields of laser control, communications, flat panel displays, scanning devices and recording and reproduction devices.
Interactions with energy beams such as optical or acoustic beams can be controlled by means of applied electric fields in electro-optic (EO) or piezoelectric materials. An electrically controlled spatial pattern of beam interaction is desired in a whole class of switched or modulated devices. Patterned responses can be achieved in uniform substrates using the electro-optic or piezoelectric effect by patterning the electric field. However, Maxwell""s equations for the electric field prevent sharp field variations from extending over a large range. Some materials can be poled, which means their electro-optical and/or piezoelectric response can be oriented in response to some outside influence. In these materials, is possible to create sharp spatial variations in EO coefficient over potentially large ranges. By combining slowly varying electric fields with sharply varying (poled) material, new types of patterned structures can be fabricated and used.
Polable EO materials have an additional degree of freedom which must be controlled, as compared to fixed EO crystals. Usually, the substrate must be poled into a uniformly aligned state before any macroscopic EO response can be observed. Uniformly poled substrates have been fabricated both from base materials where the molecules initially have no order, and from base materials where the molecules spontaneously align with each other locally, but only within randomly oriented microscopic domains. An example of the first type of material is the nonlinear polymer. Examples of the second type of material are sintered piezoelectric materials such as lead zirconate titanate (PZT), liquid crystals, and crystalline ferroelectric materials such as lithium niobate (LiNbO3). Nonlinear polymer poling is described in ♦ E. Van Tonmme, P. P. Van Daele, R. G. Baets, P. E. Lagasse, xe2x80x9cIntegrated optic devices based on nonlinear optical polymersxe2x80x9d, IEEE JQE 27 778, 1991. PZT poling is described for example in ♦ U.S. Pat. No. 4,410,823, October 1983, Miller et al, xe2x80x9cSurface acoustic wave device employing reflectorsxe2x80x9d. (Liquid crystal poling is described in standard references, such as S. Chandrasekhar, Liquid Crystals, Second Edition (1992), Cambridge University Press, Cambridge.) Ferroelectric crystal poling is described in ♦ U.S. Pat. No. 5,036,220 July 1991, Byer et al., xe2x80x9cNonlinear optical radiation generator and method of controlling regions of ferroelectric polarization domains in solid state bodiesxe2x80x9d.
Examples of poled EO devices include:
the beam diffractor in a polymer layer with interdigitated electrodes of S. Ura, R. Ohyama, T. Suhara, and H. Nishihara, xe2x80x9cElectro-optic functional waveguide using new polymer p-NAn-PVA for integrated photonic devices,xe2x80x9d Jpn. J. Appl. Phys., 31, 1378 (1992) [UOS92];
the beam modulator in a polymer layer with planar electrodes of U.S. Pat. No. 5,157,541 October 1992, Schildkraut et al. xe2x80x9cOptical article for reflection modulationxe2x80x9d;
the total internal reflection beam reflector in a lithium niobate waveguide with an electrode pair of H. Naitoh, K. Muto, T. Nakayama, xe2x80x9cMirror-type optical branch and switchxe2x80x9d, Appl. Opt. 17, 101-104 (1978);
the 2xc3x972 waveguide switch in lithium niobate with two electrodes of M. Papuchon, Am. Roy, xe2x80x9cElectrically active optical bifurcation: BOAxe2x80x9d, Appl. Phys. Lett. 31, 266-267 (1977); and
the wye junction beam router in a lithium niobate waveguide with three electrodes of H. Sasaki and I. Anderson, xe2x80x9cTheoretical and experimental studies on active y-junctions in optical waveguidesxe2x80x9d, IEEE Journ. Quant. Elect., QE14, 883-892 (1978).
These devices use uniformly poled material with varied electrode and optical structures. Many of the advantages of patterned poled devices have not been recognized. For example, in the book by
H. Nishihara, M. Haruna, T. Suhara, Optical Integrated Circuits, McGraw-Hill, New York (1989) [NHS89], many electro-optical devices activated by various electrode patterns are described, but all of these devices are fabricated on a uniformly poled substrate. The same is true of another review article, ♦ T. Suhara and H. Nishihara, xe2x80x9cIntegrated optics components and devices using periodic structures,xe2x80x9d IEEE J. Quantum Electron., QE-22, 845, (1986) [TH86], which describes the general characteristics of grating coupled devices without recognizing the advantages of a poled grating as opposed to an electrode grating.
In selected instances in the literature, certain advantages of patterned poled substrates have been pointed out.
A surface acoustic wave reflector with an array of domain reversals in a piezoelectric ceramic (but no electrodes) is described in U.S. Pat. No. 4,410,823, Miller et al.;
A beam steerer with triangular domain reversed regions in LiTaO3 is described in Q. Chen, Y. Chiu, D. N. Lambeth, T. E. Schlesinger, D. D. Stancil, xe2x80x9cThin film electro-optic beam deflector using domain reversal in LiTaO3xe2x80x9d, CTuN63, CLEO""93 Conference Proceedings, pp 196 et. seq., Optical Society of America.
A Mach-Zehnder modulator with domain reversals to compensate phase differences between microwave and optical beams is described in U.S. Pat. No. 5,278,924, January 1994, Schaffner, xe2x80x9cPeriodic domain reversal electro-optic modulatorxe2x80x9d.
A Mach-Zehnder electric field sensor with one domain reversed region in an electro-optic substrate is described in U.S. Pat. No. 5,267,336, November 1993, Sriram et al., xe2x80x9cElectro-optical sensor for detecting electric fieldsxe2x80x9d.
Use of patterned poled structures offers efficiency advantages in beam control (including generation, modulation, redirection, focussing, filtration, conversion, analysis, detection, and isolation) with applications in laser control; communications; data storage; and display. What is needed in these areas are adjustable methods for beam control with high efficiency. Due to the sharp domain transitions, higher efficiency devices can generally be obtained using pattern poled substrates to create the high frequency variations; the electrodes are needed to excite the patterned poled substrate, not to create the high frequency variations.
The poling process in polymers is quite different from that of crystals, and results in poorly defined domain boundaries. In crystals, there are a discrete number of (usually two) poling directions which are stable, and poling a local region consists of flipping atoms between these alternative states. Poled regions are fully aligned, and sharp boundaries exist between oppositely aligned domains. In poled polymers, any molecule can be oriented in any direction regardless of the poling direction. The poling process produces only an average component of alignment within a random distribution of individual molecules. In polymers, the poling (and the related EO coefficients) therefore have a continuous variation in strength and orientation. The sharp domain boundaries obtained in crystals are absent. This has a profound influence on the efficiency of certain types of poled device in polymers. Since the poling strength and direction in polymers follows the strength and direction of the local applied electric field, it is not possible to obtain poling features with spatial dimensions any sharper than permitted by Maxwell""s equations. In polymers, there is very little advantage to be obtained from spatially patterning the poled regions instead of the electrodes.
In devices based on optical polymers, poling is required to create an electro-optical response. The poling is done by applying a voltage to electrodes fabricated on the device (in the presence of heat). The entire polymer film may be poled with a uniform electrode, after which the electrodes are spatially patterned for the desired functionality. The EO performance of the device will not change much if the poling is accomplished with the patterned electrodes, since the active region within reach of the electric field is still poled almost as well. The choice of whether to pole the whole layer or just the region under the electrodes is mainly by convenience in fabrication. Examples of polymer EO devices where the poling is spatially patterned outside the active region of the device are ♦ the switched waveguides of U.S. Pat. No. 4,867,516, September 1989, Baken et al., xe2x80x9cElectro-optically induced optical waveguide, and active devices comprising such a waveguidexe2x80x9d, and ♦ U.S. Pat. No. 5,103,492, April 1992, Ticknor et al., xe2x80x9cElectro-optic channel switchxe2x80x9d. None of these devices have the electrodes traverse multiple boundaries of a patterned poled structure.
The poling process also changes the index of refraction ellipsoid in polymers. This fact has some desirable consequences, such as making possible waveguides fabricated by poling a stripe of polable polymer as described in ♦ J. I. Thackara, G. F. Lipscomb, M. A. Stiller, A. J. Ticknor, and R. Lytel, xe2x80x9cPoled electro-optic waveguide formation in thin-film organic media,xe2x80x9d Appl. Phys. Lett., 52, 1031 (1988) [TLS88] and in ♦ U.S. Pat. No. 5,006,285, April 1991, and U.S. Pat. No. 5,007,696. April 1991, Thackara et al. xe2x80x9cElectro-optic channel waveguidexe2x80x9d. However, it leaves a problem in that poled polymer boundaries are lossy in their unexcited state (they scatter, diffract and refract). Devices in which a light beam crosses poled polymer boundaries have the problem that although transparency may be achieved, the poled polymer must be activated electrically to produce a uniform index of refraction. Poled crystalline devices do not have this problem because poling does not change their index of refraction.
A solution to the problem of lack of transverse spatial definition in poled polymers was proposed in ♦ U.S. Pat. No. 5,016,959 May 1991, Diemeer, xe2x80x9cElectro-optical component and method for making the samexe2x80x9d, who describe a total internal reflection (TIR) waveguide switch in which the entire polymer film is poled, but the electro-optic coefficient of selected regions is destroyed by irradiation, creating unpoled regions with sharp spatial boundaries. While the underlying molecules in these unpoled irradiated regions remain aligned, they no longer have any electro-optic response. This approach is useful in creating sharp poled-unpoled domain boundaries in polymer films. It has the disadvantage that it cannot produce reverse poled domains so its efficiency is considerably reduced compared to the equivalent crystal poling technique.
In nonlinear frequency conversion devices, domains of different polarity are typically periodically poled into a nonlinear optic material, but not excited by an electric field. The poled structure periodically changes along the axis of the beam to allow net energy conversion despite a phase difference that accumulates between the two beams. This process is known as quasi-phasematching, and has been demonstrated in ferroelectrics [U.S. Pat. No. 5,036,220, Byer et al.] such as lithium niobate, KTP, and lithium tantalate, as well as in polymers, as described in ♦ U.S. Pat. No. 4,865,406 September 1989, Khanarian et al, xe2x80x9cFrequency doubling polymeric waveguidexe2x80x9d. Electrodes are not typically used in these devices, since the phasematching occurs in the absence of an electric field. Generalized frequency conversion in polymers is described in ♦ U.S. Pat. No. 5,061,028 October 1991, Khanarian et al, xe2x80x9cPolymeric waveguides with bidirectional poling for radiation phase matchingxe2x80x9d, as well as TE-TM modulation. Khanarian et al. used patterned electrodes in both patents to pole the polymer film; the attendant loss in sharpness of the spatial pattern becomes a severe problem where more complex electrode structures are needed such as in the latter patent.
Devices are known employing periodic structures which use electric fields to control gratings in order to control propagating fields. A diffraction grating modulator is shown in ♦ U.S. Pat. No. 4,006,963, Feburary 1977, Baues et al. xe2x80x9cControllable, electro-optical grating couplerxe2x80x9d. This structure is fabricated by removing material periodically in an electro-optic substrate to form a permanent grating. By exciting the substrate electro-optically, the fixed index grating has a greater or lesser effect, producing some tuning. This structure does not contain poled regions. The drawbacks of the Baues structure are the same as for the polymer film: the grating cannot be made transparent without the application of a very strong field.
The current technology for an EO switchable grating is shown in FIG. 1 (Prior Art). In this structure, periodically patterned electrodes serve as the elements that define the grating. The underlying material does not have a patterned poled structure, as hereinafter explained. An input beam 12 is coupled into a electro-optically active material 2 which contains an electrically controllable grating 6. When the voltage source 10 to the grating electrodes is off, the input beam continues to propagate through the material to form the output beam 16. When the grating-controlling voltage source is switched on, an index modulation grating is produced in the material, and a portion of the input beam is coupled into a reflected output beam 14. The material has an electro-optically active poled region 4 with a single domain, with the same polarity throughout the poled structure. A first electrode 6 is interdigitated with a second electrode 7 on a common surface 18 of the substrate. When a voltage is applied between the electrodes, the vertical component of electric field along the path of the beam 12 alternately has opposite sign, creating alternate positive and negative index changes to form a grating. The strength of the grating is controlled by the voltage source connected between the two electrodes by two conductors 8.
A second general problem with the existing art of EO and piezoelectric devices using uniform substrates and patterned electrodes is that the pattern of the excited electric field decays rapidly with distance away from the electrodes. The pattern is essentially washed out at a distance from the electrodes equal to the pattern feature size. This problem is aggravated in the case of a grating because of the very small feature size. Prior art gratings formed by interdigitated electrodes produce a modulated effect only in a shallow surface layer. EO structures interact weakly with waveguides whose dimension is larger than the feature size. While longer grating periods may be used in higher order interaction devices, the lack of sharp definition described above again seriously limits efficiency. The minimum grating period for efficient interaction with current technology is about 10 microns. What is needed is a way to maintain the efficiency of EO devices based on small structures, despite a high aspect ratio (i.e. the ratio of the width of the optical beam to the feature size). Switchable patterned structures are needed which persist throughout the width of waveguides and even large unguided beams.
In bulk material, gratings may be formed by holographic exposure and acoustic excitation. Holographic exposure is very difficult, and storage materials such as SBN are not yet developed to a commercial state. Acoustic excitation is very expensive to implement and to power, and requires additional components such as soft mounts and impedance matched damping structures. Other methods form surface gratings, including deposition techniques, material removal techniques and material modification techniques (such as indiffusion, outdiffusion, and ion exchange). What is needed is an approach capable of a large enough aspect ratio to produce bulk interaction structures, preferably with feature control at an accessible surface.
While the EO material can in principle be any electro-optically active material, liquid crystals are a special case and have limited applicability. A light modulator based on diffraction from an adjustable pattern of aligned liquid crystal domains is described in ♦ U.S. Pat. No. 5,182,665, January 1993, O""Callaghan et al., xe2x80x9cDiffractive light modulatorxe2x80x9d. A light modulator based on total internal reflection modulated by liquid crystal domain formation is described in ♦ U.S. Pat. No. 4,813,771 March 1989, Handschy et al., xe2x80x9cElectro-optic switching devices using ferroelectric liquid crystalsxe2x80x9d. In all of these devices, the domains must physically appear or disappear to produce the desired effect. The orientation of the molecules in the liquid crystal device changes in response to an applied field, producing a patterned structure which interacts with light. However, liquid crystals have important drawbacks. They are of course liquid and more difficult to package, and they have a limited temperature range and more complex fabrication process than solid state devices. High aspect ratio structures cannot be made because of the decay of the exciting field pattern with distance. The molecular orientation relaxes as soon as the field is turned off, and re-establishing the pattern takes a long time, so fast switching is not possible.
The structures which switch light from waveguide to waveguide in the prior art have a high insertion loss or large channel spacing which render them unsuitable for large routing structures. A large switching structure must have switching elements with insertion loss low enough to permit light to propagate through the structure. If a waveguide has 100 switches, for example, the switches must have less than about 0.03 dB insertion loss. In the prior art this is not possible. R. A. Becker and W. S. C. Chang, xe2x80x9cElectro-optical switching in thin film waveguides for a computer communications busxe2x80x9d, Appl. Opt. 18, 3296 (1979), demonstrate a multimode crossing waveguide array structure coupled via interdigitated electro-optic grating switches. This switch has an inherently high insertion loss (0.4 dB) and poor switching efficiency (≈10%). U.S. Pat. No. 5,040,864, August 1991, J. H. Hong, xe2x80x9cOptical Crosspoint Switch Modulexe2x80x9d, discloses a planar waveguide structure which may in principle have a low insertion loss, but which requires very large crossing junctions for efficient switching, and is therefore incapable of producing a high density switching array.
In summary, the prior art has shortcomings in several areas: 1) large aspect ratios of controllable patterns are needed for efficient interaction with bulk waves or small patterns; 2) sharp domain transitions are needed for efficiency in higher order interactions; 3) transparency of domain structures is needed at zero applied field for proper unpowered operation; and 4) low insertion loss is required for arrays of switches. Poled structures contained in the above and other structures have not been fully utilized heretofore to realize practical devices.
According to the invention, a method for optical energy transfer and energy guidance uses an electric field to control energy propagation using a class of poled structures in solid material. The poled structures, which may form gratings in thin film or bulk configurations, may be combined with waveguide structures. Electric fields are applied to the poled structures to control routing of optical energy. Techniques include frequency-selective switchable- and adjustable-tunable reflection, splitting, directional coupling, frequency-tunable switching and efficient beam combining, as well as polarized beam combining. Adjustable tunability is obtained by a poled structure which produces a spatial gradient in a variable index of refraction along an axis in the presence of a variable electric field.
In one embodiment, the present invention is a method of switching a grating which consists of a poled material with an alternating domain structure of specific period. When an electric field is applied across the periodic structure, a Bragg grating is formed by the electro-optic effect, reflecting optical radiation with a certain bandwidth around a center wavelength. The grating may be used by itself, or in combination with other gratings to form integrated structures in a ferroelectric crystal. Specifically of interest is an method of using an integrated structure in which one or more optical waveguides interact with one or more periodic structures to form a wavelength selective integrated optic modulator, switch, or feedback element.
The invention will be better understood upon reference to the following detailed description in connection with the accompanying drawings.